(1) Field of the Invention
The present invention relates to an optical modulator that modulates the intensity or the phase of light by using the electro-optical effect, and particularly to an optical modulator provided with an electric filter circuit for flattening and leveling the frequency characteristic of optical response.
(2) Related Art
A conventional optical modulator is configured, for example, as shown in FIG. 11. Referring to FIG. 11, an optical waveguide 101 of Mach-Zehnder type and an electric waveguide (progressive wave electrode) 102 for controlling the relative phase of each light that propagates through the two parallel arm sections 101A, 101B of the optical waveguide 101 are formed on a substrate 100 having an electro-optical effect. By applying a modulation electric signal S supplied from a driving circuit 110 to one end of the electric waveguide 102, the refractive index of one arm section 101A of the optical waveguide 101 is controlled and, by changing the optical path length difference between the two arm sections 101A, 101B, optical modulation of the input light LIN is realized. With this conventional optical modulator, the other end of the electric waveguide 102 is terminated by a resistor RT, and a DC bias VB for controlling the relative phase shift amount between the two arm sections 101A, 101B is applied to the other end of the electric waveguide 102 via a bias tee circuit 120. Such a conventional optical modulator using the electro-optical effect is used, for example, in an optical transmission system that performs a high-speed long-distance optical communication.
For the conventional optical modulator such as described above, in order to obtain output light LOUT modulated in a more preferable state, flatness of the optical response band of the optical modulator is required in a frequency region contained in the modulation electric signal. However, the driving voltage needed for realizing a suitable optical modulation and the optical response band are in a trade-off relationship with each other. Therefore, as one of the conventional techniques for restraining the rise of the driving voltage and realizing the widening of the optical response band, a configuration using a filter circuit that compensates for the dependency of modulation efficiency on the frequency, for example, is known in the art (See, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-287103).
FIG. 12 is a view showing a configuration example of a conventional optical modulator using the above-described filter circuit. In this optical modulator, a filter circuit 130 made of a capacitor C1 and a resistor R1 that are connected in parallel, for example, is inserted between the driving circuit 110 and one end of the electric waveguide 102. Typically, a chip capacitor and a thin film resistor, or a chip capacitor and a chip resistor are used as the capacitor C1 and the resistor R1.
FIG. 13 exemplifies a frequency characteristic of the reflection coefficient (S11) of an electric circuit and a frequency characteristic of the optical response in a conventional optical modulator. The top view shows a case with no filter circuit, which corresponds to the configuration of FIG. 11, and the bottom view shows a case with a filter circuit, which corresponds to the configuration of FIG. 12. Here, a driving circuit 110 of 50 Ω series is used at a bit rate of 10 Gbps, and the terminal resistance RT and the characteristic impedance Z of the electric waveguide 102 are respectively set to be 50 Ω which is equal to the impedance of the above driving circuit 110.
In order to realize a high-speed operation and a low-voltage operation in an optical modulator with no filter circuit, the length of the progressive wave electrode 102 must be sufficiently increased; however, in such a progressive wave electrode 102, it is difficult to realize a flat optical response characteristic, as shown in the right top view of FIG. 13, due to the influence of the attenuation of the microwave caused by the surface skin effect. Therefore, by attenuating the low-frequency component of the modulation electric signal S with the use of a filter circuit 130, the flatness of the optical response characteristic in a desired frequency band (for example, a frequency band of at most a little more than the half of the bit rate in a general NRZ modulation) is improved, as shown in the right bottom view of FIG. 13. In other words, a wide optical response band is realized.
However, with a conventional optical modulator such as shown in the above FIG. 12, by providing a filter circuit 130 having a transmittance characteristic that attenuates a low-frequency component, rise in the impedance of the whole electric circuit in the low-frequency region is invited, thereby provoking the deterioration of the electric reflection characteristic (S11), as shown in the left bottom view of FIG. 13. This deterioration in the electric reflection characteristic causes a multiple reflection of the electric signal, for example, between the driving circuit 110 and the filter circuit 130, thereby giving an adverse effect on the optical modulation operation.
As a countermeasure against such deterioration of the electric reflection characteristic in the low-frequency region, the above-mentioned prior art discloses a configuration in which a constant-resistance filter circuit 130′ is formed by connecting a shunt resistor RS to the parallel circuit made of the capacitor C1 and the resistor R1, so as to reduce the S11 in the low-frequency region, as shown, for example, in FIG. 14. However, such a constant-resistance filter circuit 130′ not only invites a complex configuration but also raises the following problem.
In other words, in driving of the optical modulator with the use of the electro-optical effect, a DC bias VB must be applied to the electric waveguide 102 as described above. However, when the above constant-resistance filter circuit 130′ is inserted, the DC bias VB cannot be efficiently applied to the electric waveguide 102, due to the presence of the shunt resistor RS. For this reason, in the configuration example shown in FIG. 14, an electrode 102′ for applying the DC bias VB is formed on the substrate 100, separately and independently from the electric waveguide 102 to which the modulation electric signal S′ is applied. This raises a problem of increase in the size of the optical modulator.
In addition, in a conventional optical modulator such as described above, a chip capacitor and a chip resistor are used as the components constituting the filter circuits 130, 130′. This leads to a disadvantage of inviting deterioration of the characteristic in a high frequency, aggravation of the producibility such as mounting a chip component, increase in the size of the filter circuits.